In order to detect insulation faults in continuous wires or cables, so-called spark testers are employed in which electrodes that are at a relatively high voltage lie loosely against the passing wire or cable while the conductor or conductors of the wire or cable are grounded. Generally loose chains or small wire brushes are employed as electrodes. Both types of electrodes are not fully satisfactory. In the case of chains there results an irregular and unreliable distribution of the voltage state, that is, the corona, at the surface of the insulation. In the case of metal brushes, abrasion produces very pointed and sharp-edged metal pieces that hook themselves into the insulation surface and thus may lead to short circuits. This is particularly the case when testing foamed insulations.
Prior art spark testers often have only one relay which, if there are insulation faults and sparkovers caused thereby between the electrodes and the conductor in the wire or cable, responds to the increased current and actuates a counter. This arrangement also has been found to be disadvantageous because, in order to obtain sufficient sensitivity, it has to operate with relatively high testing voltages. This necessity exists also because, under certain circumstances, considerable fluctuations may occur in the voltage. If tests are made with an alternating voltage, variable capacitances between the testing electrodes and the conductor of the object to be tested may produce considerable voltage fluctuations; be it that the voltage drops too much; be it that resonance causes undesirable excess voltages to occur. In the case of testing with direct voltage, charge transport at the surface of the insulation may produce a variable load and thus undesirable changes in voltage.